1. Field of the Invention
The present invention relates to a method of manufacturing a liquid crystal display in which a gap between substrates is filled with a liquid crystal using the one drop filling method.
2. Description of the Related Art
FIG. 5 shows a schematic configuration of a liquid crystal display panel according to the related art. FIG. 5 also shows an equivalent circuit of one pixel. As shown in FIG. 5, the liquid crystal display panel is constructed by combining an array substrate 51 constituted by a glass substrate 63 having thin film transistors (TFTs) 57 formed thereon and an opposite substrate 79 having a color filter (CF) layer (not shown) formed thereon in a face-to-face relationship using a sealing material 61 to seal a liquid crystal 75 between the substrates.
At one of the shorter ends of the glass substrate 63 used as the array substrate 51, OLB (Outer Lead Bonding) pads 69a, 69b, 69c and 69d (hereinafter briefly referred to as OLB pads 69a to 69d) are formed, driver ICs for driving a plurality of gate bus lines (only one gate bus line 53 is shown in FIG. 5) being connected to the respective pads. Each of the OLB pads 69a to 69d has a plurality of voltage supply terminals (not shown) formed substantially orthogonal to the shorter end of the glass substrate 63 at a predetermined pitch.
Lead-out wiring portions 71a, 71b, 71c and 71d (hereinafter briefly referred to as lead-out wiring portions 71a to 71d) are formed such that they extend from the OLB pads 69a to 69d toward a display area that is surrounded by the sealing material 61. The sealing material 61 is formed like a frame at the periphery of the glass substrate 63. The lead-out wiring portions 71a to 71d have a plurality of lead-out wirings (not shown) connected to the voltage supply terminals of the OLB pads 69a to 69d, respectively. A gate bus line extending in the horizontal direction in the figure is connected to each of the lead-out wirings.
At one of the longer ends of the glass substrate 63, OLB pads 65a, 65b, 65c, 65d, 65e, 65f, 65g and 65h (hereinafter briefly referred to as OLB pads 65a to 65h) are formed, driver ICs for driving a plurality of drain bus lines (only one drain bus line 55 is shown in FIG. 5) being connected to the pads. Each of the OLB pads 65a to 65h has a plurality of voltage supply terminals (not shown) formed substantially orthogonal to the longer end of the glass substrate 63 at a predetermined pitch.
Lead-out wiring portions 67a, 67b, 67c, 67d, 67e, 67f, 67g and 67h (hereinafter briefly referred to as lead-out wiring portions 67a to 67h) are formed such that they extend from the OLB pads 65a to 65h toward the display area. The lead-out wiring portions 67a to 67h have a plurality of lead-out wirings (not shown) connected to the voltage supply terminals of the OLB pads 65a to 65h, respectively. A drain bus line extending in the vertical direction in the figure is connected to each of the lead-out wirings.
The plurality of gate bus lines are formed such that they intersect the plurality of drain bus lines with an insulation film, which is not shown, interposed between them. A TFT is formed at a pixel region which is formed at each of intersections between the gate bus lines and the drain bus lines. As shown in FIG. 5, for example, a TFT 57 and a pixel electrode 59 are formed at the intersection between the gate bus line 53 and the drain bus line 55. A drain electrode 57a of the TFT 57 is electrically connected to the drain bus line 55. A gate electrode 57b of the TFT is electrically connected to the gate bus line 53. A source electrode 57c of the TFT is electrically connected to the pixel electrode 59. A glass substrate is used as the opposite substrate 79 just as done for the array substrate 51, and an opposite electrode 60 is formed on an entire surface of the glass substrate. A liquid crystal capacitance 58 is formed by the pixel electrode 59, the opposite electrode 60 and a liquid crystal 75 that is sandwiched between the pixel electrode 59 and the opposite electrode 60.
The gate bus lines 53, the drain bus lines 55 and the TFTs 57 are formed using a photolithographic technique at a step for manufacturing the array substrate 51. In the case of a liquid crystal display having a large display area, it may be difficult to transfer patterns on an entire glass substrate 63 at a time for reasons associated with the structure of the exposure apparatus used. For this reason, divisional exposure is performed, in which an entire patterned region of the glass substrate 63 is exposed after dividing it into a plurality of regions to be exposed. During divisional exposure, a predetermined exposure mask is used for each divided region to be exposed. A resist film formed on the glass substrate 63 is shield from light in regions other than the divided regions to be exposed, and each of the divided regions to be exposed is exposed using a predetermined exposure mask and is thereafter developed to form a resist pattern for the entire regions.
During divisional exposure, the exposure mask for each divided region to be exposed is aligned with the glass substrate 63. Therefore, the exposure mask for each divided region to be exposed can be misaligned with the glass substrate 63, and the width of an overlap between the source electrode 57c and the gate electrode 57b of the TFT 57 may be different in each of the divided regions to be exposed. In this case, since parasitic capacitances that are formed between the gate electrodes 57b and the source electrodes 57c of TFTs 57 in each of the divided regions to be exposed are different from those in other regions, a difference in pixel potential occurs between the divided regions to be exposed, which results in a difference in light transmittance. Therefore, differences in luminance occur on the display screen of the liquid crystal display and will be visually perceived as irregularities of display. For example, when gray is displayed on the display screen, stripe-like irregularities 74 may be visually perceived on exposure joint lines 73 at boundaries between divided regions to be exposed adjacent to each other.
The one drop filling method is known as a method of filling the gap between the array substrate 51 and the opposite substrate 79 with a liquid crystal. The one drop filling method and a liquid crystal display panel formed using the same will be described with reference to FIG. 6. For example, according to the one drop filling method, a prescribed amount of the liquid crystal 75 is dispensed onto a substrate surface inside the sealing material 61 formed like a frame at the periphery of the array substrate 51 such that the droplets form a matrix as shown in FIG. 6, and the array substrate 51 and the opposite substrate 79 are combined in vacuum. Next, the substrates 51 and 79 are returned to the atmospheric pressure, and the liquid crystal 75 is consequently spread by the atmospheric pressure. The sealing material 61 is cured while the liquid crystal 75 is spread. The liquid crystal display panel is then heated to cause the liquid crystal to flow, whereby the layer of the liquid crystal 75 sealed between the array substrate 51 and the opposite substrate 79 has a uniform thickness. The liquid crystal display panel is thus completed.
In general, the positions and number of droplets of the liquid crystal 75 are determined in consideration to the spreading property of the liquid crystal 75 and the takt time and dispensing capability of the dispenser used, and certain regularity is found in them. For example, droplets of the liquid crystal 75 may be dispensed in the form of a matrix as shown in FIG. 6 or may be dispensed in a staggered configuration in which the droplets are offset with respect to one another by half a pitch. All droplets of the liquid crystal 75 dispensed on the array substrate 51 spread in every direction at substantially the same speed when the two substrates 51 and 79 are combined. As a result, adjoining droplets of the liquid crystal 75 contact each other substantially in the middle of the gap between the respective dispensing positions. Boundaries at which adjoining droplets of the liquid crystal 75 contact each other within the surface of the array substrate 51 have grid-like contours when the liquid crystal 75 is dispensed in the form of a matrix and honey-comb-like contours when the liquid crystal is dispensed in a staggered configuration.
The display screen of the liquid crystal display formed using the one drop filling method may have display irregularities which follow the contours of the boundaries where adjoining droplets of the liquid crystal 75 contact each other. For example, let us assume that the liquid crystal 75 is dispensed in the form of a matrix as shown in FIG. 6. When gray is displayed throughout the display screen, dark grid-like irregularities 77 may be visually perceived at the boundaries where adjoining droplets of the liquid crystal 75 contact each other.
Patent Document 1: Japanese Patent Laid-Open No. JP-A-2002-341361
Patent Document 2: Japanese Patent Laid-Open No. JP-A-2002-09757
Recently, liquid crystal displays are extensively used as display screens of television receivers. When a liquid crystal display is used in a television receiver, the luminance of a backlight unit provided in the display is set higher than that in the case of the use of the display in a personal computer. Therefore, stripe-like irregularities 74 at exposure joint lines 73 become more visually perceptible and so do grid-like irregularities 77 at boundaries where adjoining droplets of the liquid crystal 75 contact each other.
The stripe-like irregularities 74 and the grid-like irregularities 77 can be made less visually perceptible by improving the accuracy of alignment between exposure masks of the divisional exposure apparatus or adjusting the positions and number of droplets of the liquid crystal 75. However, when the positions of exposure joint lines 73 are coincident with or very close to boundaries where adjoining droplets of the liquid crystal 75 contact each other as shown in FIG. 7, a synergistic effect between stripe-like irregularities 74 and grid-like irregularities 77 generates stripe-like irregularities 81 which are deeper in color than the irregularities 74 and 77. In this case, it is not possible to make the stripe-like irregularities 81 less noticeable by improving the accuracy of alignment between the exposure masks of the divisional exposure apparatus or adjusting the positions and number of droplets of the liquid crystal 75, and the display quality of the liquid crystal display will be significantly degraded by the stripe-like irregularities 81.
It is an object of the invention to provide a method of manufacturing a liquid crystal display which can achieve high display characteristics.